The cytokine interleukin-4 (IL-4) is a double-edged sword: it inhibits macrophage-mediated inflammation but promotes allergy and asthma, and it promotes host control of gastrointestinal worm infections but inhibits host protection against intracellular parasites. These contrasting disease-inhibiting and disease-promoting effects of IL-4 make appropriate regulation of IL-4 responses necessary. Although several molecules suppress IL-4 responses, only one molecule induces T cells to differentiate, in vitro, into IL-4-secreting cells: IL-4 itself. Because IL-4 priming of T cell IL-4 production depends on a signaling pathway that requires activation of Stat6, it might be anticipated that Stat6-deficient mice would produce little or no IL-4; however, studies with a new technique, that directly measures IL-4 secretion in vivo, indicate that Stat6-deficient mice make normal IL-4 responses to some antigens and decreased, but still substantial IL-4 responses to other antigens. Thus, Stat6 signaling is not required for T cells to differentiate into IL-4-secreting cells in vivo. In vivo studies also indicate that IL-4 responses to some antigens decline despite continuing antigen administration; these declines are more marked in Stat6-deficient than in wild-type mice. These observations suggest four questions: 1) If Stat6-dependent IL-4 priming is not required for the in vivo development of IL-4 responses, why are IL-4 responses to some antigens considerably smaller in Stat6-deficient mice than in wild-type mice? 2) What properties account for the ability of some antigens to stimulate similar IL-4 response in wild-type and in Stat6-deficient mice, while others stimulate considerably larger IL-4 responses in wild-type mice than in Stat6-deficient mice? 3) What causes IL-4 responses to some antigens to decrease despite continuing antigenic stimulation? and 4) Why is the latter phenomenon particularly pronounced in Stat6-deficient mice? An additional observation, that the Stat6-activating cytokine IL-13 promotes in vivo type 2 cytokine responses even though it does not bind to T cells, raises a fifth question: How does Stat6 signaling of non-T cells promote IL-4 secretion? Our preliminary observations lead us to respond to these questions by hypothesizing that: 1) IL-4 production is induced, as a default response, by strong antigenic stimulation in the absence of stimuli that inhibit IL-4 production; 2) the production of antigen- antibody complexes downregulates ongoing IL-4 production by inducing the production of cytokines that inhibit IL-4 production; 3) allergens promote IL-4 responses by acting on macrophages, dendritic cells, and/or NK cells to suppress production of the cytokines that inhibit IL-4 responses; and 4) enzymatic properties of allergens contribute to their allergenicity by inhibiting macrophage, dendritic cell, and/or NK cell production of cytokines that suppress IL-4 production. Experiments to test these hypotheses will be performed with IL-4-deficient, IL-4 receptor alpha chain-deficient, Stat6-deficient, and B cell-deficient mice, and with IL-4, IL-13, IL-12, and IFN-gamma agonists and antagonists. These experiments will utilize our new assay that allows direct measurement of in vivo IL-4 production. The results of these experiments will provide a better understanding of how IL-4 responses are regulated in vivo; this understanding will enhance the ability to control disorders that are regulated by IL-4.